Conventionally, in order to protect a motor from being burned, a protection element such as a bimetal or a PTC is built in a motor housing. When a motor abnormally generates heat, an electric circuit is interrupted by this protection element and the passage of electric current through the motor is stopped.
However, when the above-mentioned protection element is arranged near the motor, the size of the motor is enlarged to increase the size of an entire device. Therefore, in a motor controller described in JP-H11-164472-A, the above-mentioned protection element is not provided but a control section for driving and controlling the motor calculates the estimated temperature of the motor on the basis of the magnitude of voltage applied to the motor, a period of time during which voltage is applied to the motor, and the last estimated temperature. In the motor controller described in JP-H11-164472-A, when a calculated estimated temperature becomes not less than a predetermined overheat protection temperature, driving the motor is stopped and a stopping state is further kept until the estimated temperature reaches an overheat protection release temperature.
By the way, to calculate the estimated temperature of a motor after stopping the operation of the motor, an estimation equation using only a stopping time that passes while the motor is stationary as a parameter is generally used and the estimation equation does not take a change in the ambient temperature during the stopping time into account. Hence, there is presented a problem that the accuracy of the estimated temperature of the motor (winding) is not good.
Moreover, there is presented the following problem: when a first-order approximate equation is used in the processing of calculating the estimated temperature of a motor when the motor is stopped, the accuracy of the estimated temperature is degraded; however, when a try to calculate the estimated temperature with higher accuracy is made, the processing load of a control section is increased and hence an expensive microcomputer or the like needs to be used for the control section.